The present invention relates to a control device for an engine capable of using fuel into which alcohol is mixed.
Although an engine for a vehicle such as an automobile generally uses gasoline as a fuel, an engine which can use fuel into which alcohol is mixed is also well-known. A vehicle which mounts an engine capable of using fuel into which alcohol is mixed at an arbitrary mixing ratio is generally referred to as a FFV (Flexible Fuel Vehicle).
An engine for such an FFV is also, in the same manner as the gasoline engine, subject to a control where an exhaust air/fuel ratio is detected based on oxygen concentration in an exhaust gas using an oxygen sensor or an LAFS (linear air/fuel ratio sensor), and a fuel feed quantity is feedback-controlled such that the exhaust air/fuel ratio approximates a target air/fuel ratio.
Here, in the FFV, it is necessary to properly perform a control of fuel injection quantity by taking the difference in properties of both fuels such as the difference in theoretical air/fuel ratio between gasoline and alcohol into consideration. That is, it is necessary to properly grasp alcohol concentration of fuel into which alcohol is mixed and to perform a proper fuel-feed quantity control corresponding to the alcohol concentration. Here, to properly grasp the alcohol concentration, it is necessary to suitably learn alcohol concentration corresponding to a state where alcohol concentration is changeable as in the case of feeding fuel into a vehicle.
There has been known an internal combustion engine which detects properties of fuel based on a displacement amount between a basic fuel injection time which is set corresponding to a running state and an actual fuel injection time (for example, Patent Document 1).
On the other hand, there has been also proposed a technique which detects a malfunction of a fuel feeding system represented by clogging of a fuel injection valve or the like based on a change of a correction amount of a feedback control. In this case, a state where an exhaust air/fuel ratio is continuously maintained in a lean state in spite of a control of a correction amount so as to allow the exhaust air/fuel ratio to have a stoichiometric amount of air is determined as the occurrence of clogging of a fuel injection valve.
[Patent document 1] Japanese Patent No. 2956237
However, in the state of the art, it is difficult to accurately distinguish whether a change of correction amount in the feedback control is brought about by an actual change in alcohol concentration due to feeding of fuel or the like or by the displacement between a basic fuel injection quantity and an actual fuel injection quantity attributed to an malfunction of a fuel feeding system or the like. Accordingly, there may be a case where the alcohol concentration of mixed fuel cannot be accurately learnt. Since the fuel injection quantity is subject to the feedback control based on the exhaust air/fuel ratio as described above, even when the exhaust air/fuel ratio is changed due to a factor other than a change of alcohol concentration of a mixed fuel, an actual fuel injection quantity is changed. For example, even when a fuel injection quantity is decreased due to the occurrence of clogging in a fuel injection valve, an actual fuel injection quantity is increased and becomes larger than a basic fuel injection quantity.
When alcohol concentration of the mixed fuel is detected based on merely a correction amount of a fuel injection quantity, there exists a possibility that a change of the correction amount attributed to clogging in the fuel injection valve is also erroneously determined as a change of alcohol concentration. When such erroneous determination is made, the alcohol concentration of the mixed fuel cannot be accurately learnt so that there exists a possibility that the air/fuel ratio cannot be properly controlled.